Vapor phase cleaning

ABSTRACT

In a process for cleaning one or more articles in the vapor phase of an organic solvent the solvent vapor is fed into a cleaning chamber (1) wherein an absolute pressure of 200 mbar or less is maintained and the cleaning is conducted at a temperature at or above the flash point of the organic solvent. A preferred apparatus for conducting the cleaning process contains a cleaning chamber (1), two storage tanks (2 and 3), an evaporator (4), a heating device (5) and a condenser (6). These devices are connected by means of a conduit system which is equipped with a vacuum pump (7), two pumps (8 and 9) and valves (12, 13, 14, 15, 16, 17, 18, 19, 20, 21 and 22). Inlet air (11) can be fed into the cleaning chamber (1). Waste gas (10) can be removed from the apparatus by means of the vacuum pump (7).

The present invention relates to the cleaning of one or more articles inthe vapor phase of an organic solvent. This cleaning method is generallyknown as "vapor degreasing" and is often used for degreasing articles,such as metals, glass or plastic, etc.. In vapor degreasing processesthe article to be cleaned is placed in a zone of solvent vapor. Thesurface of the article has a lower temperature than the solvent vapor.The vapor condenses on the article and subjects its surface to asolvent-flushing action as it flows downward. The liquid drops arecollected and revaporized. Thus, the surface of the article iscontinually rinsed with distilled solvent until at least the surface ofthe article has the same temperature as the solvent vapor andcondensation ceases. Very effective cleaning of the surface of thearticle is achieved. Typically halogenated solvents, such asperchloroethylene, trichloroethylene, 1,1,1 -trichloroethane ormethylene chloride are used. However, for environmental reasons the useof halogenated solvents becomes less and less desirable in spite oftheir many good properties, such as excellent cleaning efficiency,non-flammability etc.. Much research is being spent on the replacementof chlorinated solvents by environmentally more friendly solvents.However, the utility of other solvents is limited because manyhalogen-free solvents have a flash point and, accordingly, are asubstantial explosion and fire hazard.

Accordingly, one object of the present invention is to provide anefficient process for cleaning articles wherein halogen-free solventscan be used but wherein a substantial explosion hazard can be avoided.Another object of the present invention is to provide such a cleaningprocess wherein a substantial explosion hazard can be avoided by othermeans than expensive explosion proof installations or the use of inertgases.

It has been found that a substantial explosion hazard can be avoidedwhen a vapor phase cleaning is conducted in an apparatus wherein anabsolute pressure of 200 mbar or less is maintained.

EP-A-0,381,887 describes a process, wherein freon or trichloroethyleneis used as a solvent. A decreased pressure is recommended in thecleaning tank, such that no solvent vapor is released from the cleaningtank. However, the use of halogenated solvents is undesirable for theabove-mentioned reasons.

WO-A-93/08933 relates to a process wherein an object to be cleaned isplaced in a chamber and the chamber is evacuated in order to remove airand other non-condensible gases, before a solvent is introduced to thechamber. The chamber is evacuated in order to prevent that solvent ismixed with air and has to be separated from air at a later stage. Thevacuum pump is then shut off. When solvent is introduced into thechamber, the pressure in the chamber increases. Aldehydes, alcohols,amines, ketones and aromatic solvents are mentioned in addition tohalogenated solvents. However, it is not indicated how to avoid anexplosion hazard when such solvents are used.

Accordingly, one aspect of the present invention is a process forcleaning one or more articles in the vapor phase of an organic solvent,which process is characterized in that solvent vapor is fed into acleaning chamber wherein an absolute pressure of 200 mbar or less ismaintained and the cleaning is conducted at a temperature at or abovethe flash point of the organic solvent.

Another aspect of the present invention is an apparatus for conductingthe process of the present invention which comprises a cleaning chamber,an evaporator and a vacuum pump.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole FIGURE is a schematic illustration of a preferred embodiment ofthe apparatus of the invention.

It has been found that according to the process of the present inventionone or more articles can be safely cleaned in the vapor phase of anorganic solvent, even when the cleaning is conducted at a temperature ator above the flash point of the organic solvent. For the sake ofconvenience, the following description relates to the cleaning of"articles" although the process of the present invention is not limitedto the cleaning of several articles but is equally useful for cleaning asingle article. The flash point of an organic solvent is generallymeasured at atmospheric pressure. The flash point as defined hereinmeans the lowest temperature of the solvent at which the mixture ofsolvent vapor and air above the solvent can be ignited according tostandard procedures according to DIN 51755, DIN 51758 or DIN 53213. Inthe event of an explosion of the organic solvent the resulting pressureis not more than about 8 times the original pressure in the cleaningchamber. By maintaining an absolute pressure of 200 mbar or less,preferably of 125 mbar or less, more preferably of 100 mbar or less inthe cleaning chamber it is not necessary to conduct the cleaning in anexpensive apparatus which withstands high pressures or which containsexpensive explosion proof instrumentation. For economical reasons thecleaning process of the present invention is conducted in such a mannerthe absolute pressure in the cleaning chamber generally is not less than1 mbar, preferably not less than 10 mbar and most preferably not lessthan 40 mbar. By the indicated pressure is meant the prevailing pressureduring the vapor phase cleaning operation.

In addition to the described safety advantages, it was found that by theprocess of the present invention very clean articles can be obtained andthe cleaned articles can be dried rapidly and thoroughly in a veryefficient way. In order to achieve most efficient cleaning andsubsequent drying of the articles, it is essential to clean the articlesat a temperature at or above the flash point of the organic solvent. Ifthe temperature is too low, cleaning is less efficient and an incompletedrying results or the drying of the cleaned articles lasts undesirablylong. Within the given pressure limits the process of the presentinvention is preferably conducted at a temperature of at least 10° C.,more preferably at least 20° C. above the flash point of the organicsolvent. Preferably, the process of the present invention is conductedat a temperature up to 120° C., more preferably up to 100° C., mostpreferably up to 80° C.

At least some of the cleaning in the cleaning process of the presentinvention is conducted in the vapor phase of an organic solvent. Theterm "an organic solvent" as used herein encompasses undiluted organiccompounds as well as mixtures of two or more organic compounds which aregenerally designated in the art as organic solvents and also mixtures ofone of more such organic compounds with water. If a solvent mixture isused, the mixture preferably contains more than 50%, more preferablymore than 70%, most preferably more than 95% of non-halogenated organicsolvents, based on the total weight of the mixture. Most preferably, anentirely halogen-free organic solvent is used for cleaning. If the usedsolvent mixture contains water, it preferably contains less than 80%,more preferably less than 50%, most preferably less than 30% water,based on the total weight of the mixture. The cleaning process of thepresent invention is particularly useful for an organic solvent whichhas a flash point which is lower than its boiling point at atmosphericpressure and which has a boiling point of 100° C. or less at an absolutepressure of 1 mbar or more. Preferred are aliphatic hydrocarbonscontaining from 5 to 15 carbon atoms, such as cyclic saturatedhydrocarbons and linear or branched saturated or unsaturatedhydrocarbons, preferably cycloalkanes, n-paraffins, isoparaffins orStoddard solvent, or aromatic hydrocarbons, such as toluene or xylene,or oxygen-containing organic compound, such as alcohols, preferablyisopropanol, esters, preferably alkyl lactates or dibasic esters, suchas commercially available mixtures of dibasic esters, ethers, preferablydiethyl ether, ketones, preferably acetone or methyl ethyl ketone, orhydroxyethers, preferably alkoxy propanols or alkoxy ethanols, cyclicsiloxanes containing preferably 6 to 8 ring atoms or a mixture of two ormore of such compounds. The solvents which are most preferably used inthe process of the present invention have flash points in the range from10° C. to 100° C., preferably from 40° C. to 100° C.

In the practice of cleaning articles in a cleaning chamber in the vaporphase of an organic solvent, the entire procedure usually comprises thefollowing steps:

a) loading the cleaning chamber with the articles to be cleaned andclosing the cleaning chamber;

b) optionally regulating the pressure in the cleaning chamber to 200mbar or lower, more preferably to 125 mbar or lower, most preferably to100 mbar or lower and prepurifying the articles with a liquid solvent;

c) reducing the pressure in the cleaning chamber to 200 mbar or lower,more preferably to 125 mbar or lower, most preferably to 100 mbar orlower;

d) feeding solvent vapor to the evacuated cleaning chamber whereby notexceeding an absolute pressure of 200 mbar in the cleaning chamber andcleaning the articles by condensation of solvent vapor on the articles;

e) reducing the solvent vapor concentration in the cleaning chamber anddrying the cleaned articles and;

f) increasing the pressure in the cleaning chamber and unloading thecleaning chamber.

Step a) can be conducted in a known manner. The articles can for examplebe placed in containers such as baskets etc..

Step b) is optional and can also be conducted in a known manner. Vacuumpumps for achieving the desired pressure reduction are known in the artand not described in more detail herein. For prepurifying the articles,the cleaning chamber is preferably flooded with a liquid solvent. In apreferred method of flooding the cleaning chamber liquid solvent ispumped from a storage tank into the cleaning chamber. After havingcleaned the articles, the solvent is preferably returned from thecleaning chamber to the storage tank. If desired, the steps of floodingthe cleaning chamber with liquid solvent, cleaning the articles andremoving liquid solvent from the cleaning chamber can be repeated onceor more. In this case preferably fresh liquid solvent is fed fromanother storage tank to the cleaning chamber. Pumps for filling andemptying the cleaning chamber are known in the art. If the pressurereached in step b) is higher than 125 mbar, the temperature of theliquid solvent is preferably regulated that it is at least 15° C. lowerthan the flash point of the solvent. Generally, the temperature of theliquid solvent which is fed into the cleaning chamber is at least 10° C.lower than, preferably at least 20° C. lower than the temperature of thesolvent vapor which is fed into the cleaning chamber in step d). Asindicated above, the prepurifying with liquid solvent is optional. Whenthe articles are prepurified with liquid solvent, it is generallyadvisable to adjust the pressure in the cleaning chamber in two steps,i.e. prior to and after the prepurifying. When no prepurifying step isconducted, the cleaning chamber can generally be evacuated in a singlestep prior to feeding of solvent vapor into the cleaning chamber.

The evacuation procedure in step c) can be conducted in a known manner).The desired final absolute pressure in the cleaning chamber prior tofeeding the solvent vapor is equal to or less than the absolute pressureof the vapor which is fed into the cleaning chamber in step d).

In the vapor phase cleaning step d) solvent vapor is fed into thecleaning chamber wherein the absolute pressure does not exceed 200 mbar,preferably not 125 mbar, more preferably not 100 mbar. Preferably, thesolvent vapor is generated in an evaporator and fed to the cleaningchamber. Preferably, the absolute pressure in the evaporator is equal toor higher than the pressure in the cleaning chamber prior to feeding ofthe solvent vapor. However, the absolute pressure in the evaporator doesnot exceed 200 mbar, preferably not 125 mbar, most preferably not 100mbar. Preferably, the solvent vapor has a temperature at or above theflash point of the organic solvent which is used. The articles to becleaned generally have an initial temperature that is lower than thetemperature of the solvent vapor. Preferably, they have a temperaturebetween room temperature and 10° C. below the temperature of the solventvapor, more preferably between room temperature and 20° C. below thetemperature of the solvent vapor. This lower temperature causescondensation of at least a portion of the solvent vapor on the surfaceof the articles. Usually the temperature of the articles increasesduring the vapor phase cleaning, depending on the heat transfer betweenthe vapor phase and the articles. Upon completion of the vapor phasecleaning step d) at least the surface of the articles generally hasabout the same temperature as the solvent vapor. Excess solvent vaporcan be removed from the cleaning chamber and can for example becondensed in a condenser in a known manner. The condensed solvent can berecovered and further processed. For example, condensed solvent vaporcan be transferred into the evaporator or into one or more storage tanksfor further usage. The cleaning step d) is generally completed within 30minutes, typically within 5 minutes and in most cases even within 3minutes.

After the vapor phase cleaning step d) the cleaned articles aregenerally subjected to a drying operation. The pressure in the cleaningchamber is advantageously reduced. The pressure during the drying steppreferably is 1/2, more preferably 1/5, most preferably 1/10 of thepressure that is maintained during the cleaning step d). The pressurereduction facilitates rapid evaporation of excess condensed solventwhich adheres to the surface of the cleaned articles. It has been foundthat the drying is even more efficient if the pressure reduction isperformed very rapidly, e.g. by rapid opening of a connection, such as avalve, between the cleaning chamber and an evacuated container. Thedecreased solvent vapor pressure also avoids undesirably high solventemissions during unloading of the cleaning chamber. The removed solventvapors can for example be condensed in a condenser and/or adsorbed in aknown manner. The condensed and/or adsorbed solvent can be recovered andfurther processed. For example, condensed solvent vapor can betransferred into the evaporator or into one or more storage tank(s) forfurther usage.

The apparatus for conducting the process of the present inventioncomprises a cleaning chamber, an evaporator and a vacuum pump. Thecleaning chamber and evaporator should be evacuable, i.e. they should beconstrued in such a manner that they can be evacuated. The evaporatorserves for heating the organic solvent to generate solvent vapor underreduced pressure. The vacuum pump serves for evacuating the cleaningchamber and the evaporator. Advantageously, the apparatus of the presentinvention also comprises a condenser. One function of the condenser isthe condensation of excess solvent which is removed from the cleaningchamber in the above-described vapor phase cleaning step d) and/ordrying step e). An optional alternative function of the condenser incombination with the evaporator is the distillation of the organicsolvent. A preferred embodiment of the apparatus additionally comprisesone or more storage tanks for liquid solvent. The storage tank(s) shouldbe evacuable, i.e. they should be construed in such a manner that theycan be evacuated. The storage tank(s) can be connected with the cleaningchamber in a known way. Preferably, the condenser, if present, is alsoconnected with the storage tanks(s). The presence of a storage tankallows prepurifying of the articles with liquid solvent and collectionof condensed solvent from the condenser and/or from the cleaningchamber. The apparatus also contains a conduit system equipped withvalves which is not discussed in detail.

A preferred embodiment of the process and of the apparatus of thepresent invention are described in more detail with reference to thedrawing. The drawing is a schematic illustration of a preferredembodiment of the apparatus of the present invention.

The apparatus contains a cleaning chamber 1, two storage tanks 2 and 3,an evaporator 4, a heating device 5 and a condenser 6. They areconnected by means of a conduit system which is equipped with a vacuumpump 7, two pumps 8 and 9 and valves 12, 13, 14, 15, 16, 17, 18, 19, 20,21 and 22. Inlet air 11 can be fed into the cleaning chamber 1. Wastegas 10 can be removed from the apparatus by means of the vacuum pump 7.

Before the cleaning apparatus is ready for operation, liquid solvent isfilled into the evaporator 4. All valves are closed. Then valves 13, 15,17 and 22 are opened for evacuating the entire apparatus, e.g. by meansof the vacuum pump 7. When the desired pressure is reached, distillationof the liquid solvent in the evaporator 4 is started. Valves 13 and 17are closed and valves 15 and 16 are opened. The heating device 5 is setinto operation for evaporating the solvent. The solvent vapor istransmitted to the condenser 6. The condensed solvent flows into thestorage tank 2. The overflow of the storage tank 2 flows into thestorage tank 3. If needed the evaporator is fed with liquid solvent fromthe storage tank 3; for this purpose valve 21 is opened and closed asneeded. During the distillation the pressure in the cleaning apparatuscan be controlled by means of the vacuum pump 7 and alternating openingand closing of valve 22.

The cleaning apparatus is then ready for operation. Valve 12 is openedand inlet air 11 is fed into the cleaning chamber 1 until atmosphericpressure is reached in the cleaning chamber. In a first step a) thecleaning chamber is opened, loaded with articles to be cleaned andclosed again. In a second step b) valve 12 is closed and valves 13 and22 are opened for evacuating the cleaning chamber to the desiredpressure by means of the vacuum pump 7. In the prepurifying step c)valve 19 is opened and liquid solvent is pumped from the storage tank 3into the cleaning chamber 1 by means of pump 9. The efficiency of thecleaning with liquid solvent can be increased by mechanical agitation ofthe objects and/or generating ultrasonic waves in the cleaning chamber.Valve 19 is closed. When this washing procedure is finished valve 18 isopened and the contaminated liquid solvent is transmitted into thestorage tank 3. The contaminated liquid solvent can be fed into theevaporator 4 which is still operating. Valve 18 is closed. In order toperform a second cleaning operation with liquid solvent, valve 20 isopened and liquid solvent is pumped from the storage tank 2 into thecleaning chamber 1 by means of pump 8. Valve 20 is closed. When thesecond washing procedure is finished valve 17 is opened and thecontaminated liquid solvent is transmitted into the storage tank 2 fromwhere it overflows into the storage tank 3. During and after theprepurifying step c) the pressure in the cleaning chamber 1 can becontrolled by means of the vacuum pump 7 and alternating opening andclosing of the valve 22. During steps a)-c) described above thedistillation of liquid solvent in the evaporator 4 continues.

In order to start the vapor phase cleaning step d), valves 15 and 17 areclosed and valve 14 is opened. The distillation of liquid solvent isthereby interrupted. Via the opened valve 14 solvent vapor is fed intothe cleaning chamber where it condenses on the articles until theirsurface reaches the temperature of the solvent vapor. Valve 14 is thenclosed and valves 15 and 17 are opened. Condensed solvent flows into thestorage tank 2.

Prior to the drying step e) valves 15, 16 and 17 are closed. Thepressure in the cleaning chamber 1 is further lowered by means of thevacuum pump 7 and alternating opening and closing of the valve 22.Thereby the cleaned articles are dried. After the drying step e) thepressure in the cleaning chamber is adjusted to the pressure in theother parts of the cleaning apparatus by closing valve 22 and feeding acontrolled amount of fresh air into the cleaning chamber via valve 12.Then valves 15 and 16 are opened to continue the distillation of liquidsolvent. Valve 22 is opened and closed as needed to maintain the desiredpressure in the cleaning apparatus.

In step f) valve 13 is closed and valve 12 is opened again. The pressurein the cleaning chamber is thereby increased to atmospheric pressure.The cleaning chamber is opened for unloading. Then a new cleaning cyclecan be started at step a) above.

What is claimed is:
 1. A process for cleaning one or more articles inthe vapor phase of an organic solvent having a flash point of from 40°C. to 100° C., comprising the steps of feeding solvent vapor into acleaning chamber, wherein an absolute pressure of 200 mbar or less ismaintained, and cleaning of said one or more articles at a temperatureat or above the flash point of the organic solvent and a pressure of 200mbar or less.
 2. The process of claim 1, wherein an absolute pressure of125 mbar or less is maintained in the cleaning chamber.
 3. The processof claim 1, wherein the organic solvent contains more than 50%, based onthe total weight of the organic solvent, of an aliphatic hydrocarboncontaining 5 to 15 carbon atoms, an aromatic hydrocarbon, anoxygen-containing organic compound, a cyclic siloxane or a mixture oftwo or more of such compounds.
 4. The process of claim 2, wherein theorganic solvent contains more than 50%, based on the total weight of theorganic solvent, of an aliphatic hydrocarbon containing 5 to 15 carbonatoms, an aromatic hydrocarbon, an oxygen-containing organic compound, acyclic siloxane or a mixture of two or more of such compounds.
 5. Theprocess of claim 3, wherein the organic solvent contains more than 50%of an alkoxy propanol or a mixture of two or more alkoxy propanols,based on the total weight of the organic solvent.
 6. The process ofclaim 4, wherein the organic solvent contains more than 50% of an alkoxypropanol or a mixture of two or more alkoxy propanols, based on thetotal weight of the organic solvent.
 7. The process of claim 1, whereinthe organic solvent is halogen-free.
 8. The process of claim 2, whereinthe organic solvent is halogen-free.
 9. The process of claim 3, whereinthe organic solvent is halogen-free.
 10. The process of claim 1, whereinthe organic solvent is a solvent mixture containing less than 50% ofwater, based on the total weight of the solvent mixture.
 11. The processof claim 3, wherein the organic solvent is a solvent mixture containingless than 50% of water, based on the total weight of the solventmixture.
 12. The process of claim 9, wherein the organic solvent is asolvent mixture containing less than 50% of water, based on the totalweight of the solvent mixture.
 13. The process of claim 10, wherein thewater content in the solvent mixture is less than 30%.
 14. The processof claim 12, wherein the water content in the solvent mixture is lessthan 30%.
 15. The process of claim 1, wherein said one ore more articlesare pre-cleaned with a liquid solvent having a flash point of from 40°C. to 100° C. at a pressure of 200 mbar or less.
 16. The process ofclaim 9, wherein said one ore more articles are pre-cleaned with aliquid solvent having a flash point of from 40° C. to 100° C. at apressure of 200 mbar or less.
 17. The process of claim 15, wherein saidone or more articles are pre-cleaned with the liquid solvent at apressure of 125 mbar or less.
 18. The process of claim 16, wherein saidone or more articles are pre-cleaned with the liquid solvent at apressure of 125 mbar or less.
 19. The process of claim 1, wherein aftersaid cleaning step said one or more articles are subjected to a dryingoperation wherein the absolute pressure in the cleaning chamber isreduced to 1/2 or less of the pressure maintained during the cleaningstep.
 20. The process of claim 9, wherein after said cleaning step saidone or more articles are subjected to a drying operation wherein theabsolute pressure in the cleaning chamber is reduced to 1/2 or less ofthe pressure maintained during the cleaning step.